Process for producing a steel strip

ABSTRACT

A process for producing a strip-cast, low-carbon, partly Mn/Si killed steel strip which is largely free of cracks and surface defects from a steel melt having a sulfur content of between 20 and 300 ppm and an Mn/Si ratio ≧3.5, using a roll separating force of between 2 and 50 kN/m.

The invention relates to a process for the continuous production of asteel strip using at least two casting rolls and, if appropriate,laterally arranged side plates, wherein a casting reservoir, from whichliquid steel melt can be introduced to the casting rolls, can be formedbetween the casting rolls and the side plates during operation.

BACKGROUND OF THE INVENTION

During the production of a steel strip from a low-carbon, partly Mn/Sikilled steel melt, the steel strip produced, when the two-roll castingprocess known from the prior art is used, has many cracks and surfacedefects which significantly reduce the quality of the steel stripproduced.

It is known from WO03024644 and US2005145304 to prevent or at leastreduce the number of cracks and surface defects by selecting thecomposition of a steel melt in such a way that liquid non-metallicinclusions are produced in the steel melt, and these remain liquidduring the solidification of the steel shell and permit a homogeneousheat flow and therefore a homogeneous cooling effect to be achieved byforming a liquid film on the surface of the casting rolls.

-   During melting operation on an industrial scale, the MnO/SiO₂ ratios    actually present in a partly Mn/Si killed steel melt are often    substantially lower than those calculated theoretically due to    operational reasons. The melting temperature of the non-metallic    inclusions in partly Mn/Si killed steel melts is very sensitive to    changes in the steel composition and to associated changes in the    MnO/SiO₂ ratio in the composition of said inclusions. When observing    the metallurgical rules specified in the prior art for producing the    liquid, non-metallic inclusions, it is therefore not possible to    assume, during melting operation on an industrial scale, that each    treated ladle has a composition which ensures that liquid,    non-metallic inclusions are present during the casting process.    Cracks and surface defects can therefore reappear.

OBJECT OF THE INVENTION

The object of the present invention is to avoid these known drawbacks ofthe prior art, and to provide a process for producing a steel strip,which is largely free of cracks and surface defects and has ahomogeneous surface, from a low-carbon, partly Mn/Si killed steel melt.In this process, the tolerance of the melting temperature ofnon-metallic inclusions to deviations from a desired value of the steelcomposition should be sufficient to ensure that liquid, non-metallicinclusions are present in each treated ladle during the casting processduring melting operation on an industrial scale.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the object of the invention is achieved bymeans of a process in which a steel melt with a particular Mn/Si contentratio and with a particular sulfur content is processed, during normaloperation, using a particular roll separating force (RSF).

The invention therefore relates to a process for producing a strip-cast,low-carbon, partly Mn/Si killed steel strip, wherein a steel melt isintroduced from a melt reservoir between at least two casting rolls,that are cooled and move together with a steel strip, and at leastpartly solidifies on the casting rolls to form the steel strip,characterized in that the steel melt has a sulfur content of between 20and 300 ppm and an Mn/Si ratio ≧3.5 and, during normal operation, theroll separating force is between 2 and 50 kN/m.

A steel strip produced in this way is unexpectedly largely free ofcracks and surface defects and has a homogeneous surface.

A low-carbon steel strip is to be understood as meaning a steel stripwith a carbon content of less than 0.1% by weight.

The composition of the steel melt according to the invention ensuresthat the non-metallic inclusions have a low melting temperature. The lowmelting temperature has the effect that the non-metallic inclusions arepresent in a liquid state during the solidification of the steel shellon the casting rolls during the casting process. The tolerance of themelting temperature of non-metallic inclusions to deviations from adesired value of the steel composition is increased by the broadening ofthe composition range in which liquid, non-metallic inclusions arepresent in the multiphase system. This broadened composition rangeensures that the steel melt has a composition which guarantees liquid,non-metallic inclusions during the casting process even when the desiredvalue for a particular steel composition is not exactly met duringmelting operation on an industrial scale.

During the preparation of steel, oxidic or sulfidic non-metallicinclusions are produced in a steel melt. The main components of thenon-metallic inclusions in partly Mn/Si killed steel melts are MnO andSiO₂.

The setting of the sulfur content to values of between 20 and 300 ppmand of the Mn/Si ratio to values ≧3.5, in accordance with the invention,has the effect that the non-metallic inclusions are principally composedof a multiphase system having the main components MnO—SiO₂—MnS. If theMnS content of this multiphase system is less than 37% by weight MnS,the melting temperature of the multiphase system is less than themelting temperature of a multiphase system composed of the maincomponents MnO and SiO₂. The 3-phase system MnO—SiO₂—MnS has a ternaryeutectic at approximately 1130° C.

The modeling of the 3-phase system MnO—SiO₂—MnS in FIG. 1 shows that theliquidus range meets the binary boundary system MnO—SiO₂ at the eutectictemperature of 1251° C. of the latter at the eutectic point, and expandsduring the transition to a 3-phase system with an increasing MnScontent. At lower temperatures, the liquidus range moved away from theboundary system and can still be found only above certain minimum MnScontents.

Typical operating points, which simultaneously have a low meltingtemperature of the non-metallic inclusions and a tolerance of themelting temperature to fluctuations in the MnS content which issufficient during melting operation on an industrial scale, areapproximately 15% by weight MnS in the case of the composition of thesteel melt according to the invention.

The simulation of the solidification conditions in a thin-strip castinginstallation using immersion tests at inert gas, contact time andoverheating levels corresponding to strip casting and with sulfurcontents of the steel melt of between 150 and 500 ppm resulted in meanMnS contents of the liquid, non-metallic inclusions of between 7 and 40%by weight. Increased sulfur contents of partly Mn/Si killed steel meltslead to increased MnS contents of the non-metallic inclusions.

FIG. 2 shows the influence of the sulfur content of a low-carbon, partlyMn/Si killed steel melt (0.05% by weight C; 0.7% by weight Mn; 0.2% byweight Si) with an Mn/Si ratio ≧3.5 on the tendency to cracking,expressed by the frequency of cracking or by the width of the meltinginterval of the steel melt, in relation to the composition ofnon-metallic inclusions and in relation to the melting temperatures(liquidus temperatures) of the non-metallic inclusions. The measureddata in FIG. 2 were obtained from the above-mentioned immersion tests.

Below a sulfur content of the melt which leads to an MnS content of thenon-metallic inclusions which corresponds to the ternary eutectic atapproximately 1130° C., the melting temperature of the non-metallicinclusions decreases with an increasing sulfur content.

-   Above a sulfur content of the melt which leads to an MnS content of    the non-metallic inclusions which corresponds to the ternary    eutectic at approximately 1130° C., the melting temperatures of the    non-metallic inclusions and also the frequency of cracking increase    rapidly.-   The width of the melting interval increases up to a sulfur content    of approximately 300 ppm, and then remains approximately constant.

FIG. 2 shows the relative behavior between increasing tendency to hotcracking and decreasing melting temperature of the non-metallicinclusions. The sulfur content which is recommended according to theinvention and at which sufficiently low melting temperatures of thenon-metallic inclusions and simultaneously a tolerable tendency to hotcracking are achieved can therefore be derived from FIG. 2. The presenceof sulfur in a steel alloy expands the solid/liquid 2-phase area, i.e.the melting interval, of the steel alloy while simultaneously reducingthe solidus temperature thereof, and this expands the temperature rangein which hot cracks are produced between the liquid impenetrationtemperature (LIT) and the zero ductility temperature (ZDT).

The width of the 2-phase area increases approximately linearly up toapproximately 45° C. at a sulfur content of up to 300 ppm in the steelmelt. Above this sulfur content, the width of the 2-phase area remainsapproximately constant because MnS precipitates during thesolidification with an increasing sulfur content. These MnS precipitatesare deposited in solid form on the surfaces of the casting rolls andthereby prevent a homogeneous heat flow or a homogeneous cooling effect,and this encourages the formation of surface defects and cracks. Anincreasing sulfur content of the steel melt leads to increasingquantities of MnS precipitates and therefore to an increase in thenumber of surface defects and cracks.

-   According to the invention, the maximum sulfur content is therefore    limited to 300 ppm.

At a sulfur content of the steel melt of less than 20 ppm, the loweringof the melting temperature of the liquid, non-metallic inclusionscompared to multiphase systems composed of the main components MnO andSiO₂ is not large enough to ensure that liquid, non-metallic inclusionsare present during the solidification of the steel shell on the castingrolls during the casting process.

In addition, at a sulfur content of less than 20 ppm, the width of thecomposition range in which liquid, non-metallic inclusions are presentin the multiphase system is not large enough to ensure that there is asufficient tolerance to deviations from a desired value of the steelcomposition during melting operation on an industrial scale.

The sulfur content is preferably at least 50 ppm, particularlypreferably at least 70 ppm. The upper limit of the sulfur content ispreferably 250 ppm, particularly preferably 200 ppm. The sulfur contentof the steel melt can be adjusted to the desired level bydesulfurization or by the controlled addition of sulfur or of sulfurcompounds.

At an Mn/Si ratio of less than 3.5 in the steel melt, no multiphasesystem which is composed of the main components MnO—SiO₂—MnS and has asufficient reduction of the melting temperature of the liquid,non-metallic inclusions to values below the melting temperature of thesteel mixture compared to a multiphase system composed of the maincomponents MnO and SiO₂ is formed. According to the invention, the Mn/Siratio therefore needs to be greater than or equal to 3.5.

The roll separating force is the force with which the casting rolls arepressed against one another during the casting process, based on thewidth of the steel strip. The roll separating force influences thepresence of cracks and surface defects in a strip-cast steel strip.

The greater the roll separating force, the more temperatureinhomogeneities that occur at the kissing point of the steel shells.Temperature inhomogeneities of this type result in non-uniform coolingof the steel strip, and this can result in surface cracks. In addition,large roll separating forces mean that stresses are built up in thestrip-cast steel strip, and these stresses can also result in cracks andimpaired mechanical properties.

The use of a small roll separating force avoids these problems andadditionally affords the advantage that the casting apparatus issubjected to less mechanical stress. However, the selection of a smallroll separating force may adversely affect the stability of the castingprocess since, in the case of a small roll separating force, there isthe risk that the metal shells solidified on the casting rolls areinsufficiently pressed together owing to inhomogeneities during thesolidification and the steel strip cracks under its own weight, that thesteel shells remain adhered to parts or over the whole width of thecasting roll, and that cracks occur in the steel shell.

-   In processes according to the prior art, the magnitude of the roll    separating force, during normal operation, is between 5 and 250    kN/m.

According to the invention, the roll separating force is less than 50kN/m. Since the composition of the steel melt according to the inventionminimizes the occurrence of inhomogeneities during the solidification ofthe steel shells owing to the fact that it ensures the occurrence ofliquid, non-metallic inclusions, such a small roll separating force canbe used without risking the stability of the casting process.

The frequency of cracking increases with an increasing roll separatingforce. When roll separating forces of more than 50 kN/m are used, it isnot possible to ensure the production of a homogeneous surface of thesteel strip which is largely free of cracks and surface defects.

According to the invention, the lower limit for the roll separatingforce is 2 kN/m. Sufficient stability of the casting process is notensured below this value.

-   The roll separating force is preferably at least 5 kN/m. The upper    limit thereof is preferably 30 kN/m.-   The stated values for the roll separating force refer to the    steady-state normal operation of a casting installation, but not to    the conditions when the installation is starting up or when    extraordinary load effects temporarily occur.

According to a further preferred embodiment of the process according tothe invention, the non-metallic inclusions in the steel melt have a massfraction of Al₂O₃ of less than 45% by weight. The resultant multiphasesystem with the main components MnO—SiO₂—MnS—Al₂O₃ has a meltingtemperature which is less than the melting temperature of a multiphasesystem composed of the main components MnO and SiO₂. In addition, thecomposition range in which liquid, non-metallic inclusions are presentis broader in the multiphase system with the main componentsMnO—SiO₂—MnS—Al₂O₃ than in the multiphase system composed of the maincomponents MnO and SiO₂. The Al₂O₃ content is set by selecting thestarting materials for producing the steel melt and, if appropriate, bythe targeted addition of Al or Al compounds.

1. A process for producing a strip-cast, low-carbon, partly Mn/Si killed steel strip, comprising introducing a steel melt between at least two casting rolls that are cooled and that move together with a steel strip, such that the steel strip at least partly solidifies on the casting rolls to form the steel strip, and selecting the steel melt with a sulfur content of between 20 and 300 ppm and an Mn/Si ratio ≧3.5 and, during normal operation, applying a roll separating force at the casting rolls of between 2 and 50 kN/m.
 2. The process as claimed in claim 1, wherein the steel melt has a sulfur content of between 50 and 250 ppm.
 3. The process as claimed in claim 1, wherein the roll separating force is between 5 and 30 kN/m.
 4. The process as claimed in claim 1, wherein the steel melt contains non-metallic inclusions with a mass fraction Al₂O₃ of less than 45% by weight.
 5. The process as claimed in claim 1, wherein the steel melt has a sulfur content of between 70 and 200 ppm. 